DESCRIPTION (from applicant's abstract) Neurons and their targets exchange information of many sorts as synapses are formed, maintained, and modified. The investigators are using the skeletal neuromuscular junction, as the best studied of all synapses, to identify and characterize some of the target-derived cues that tell axons where, when, and how to form nerve terminals. Previous studies from this laboratory demonstrated that some of these cues are contained within the basal lamina (BL) that traverses the synaptic cleft, and suggest that others are associated with the muscle fiber membrane and the perisynaptic interstitial matrix. Subsequently, immunochemical methods were used to identify several candidate cues in the BL, membrane, and matrix. Recently, the investigators obtained evidence that one synaptic BL molecule, s-laminin/laminin beta-2 (a homologue of the laminin B1/beta 1 chain) is recognized by motoneurons in vitro and serves as a muscle-derived promoter of presynaptic deafferentation in vivo. The investigators will now extend this work to obtain a more complete picture of how beta-2 works, and how it interact with other signaling molecules to guide the transformation of a growing motor axon into a nerve terminal. The specific aims are to : (1) Elucidate the functions of the laminins and collagens of a synaptic BL , including laminin beta-2, a synapse-specific laminin alpha chain, and the collagen alpha 3-5(IV) chains, which we have found to be concentrated in synaptic BL. (2) Learn how the laminins and collagens of synaptic BL assemble into cleft material and are targeted to synaptic sites. (3) Isolate cellular receptors for synaptic BL components, so that we can learn how they convey information to the pre- and postsynaptic cells. (4) Identify components of the muscle cell membrane and perisynaptic matrix that, together with synaptic BL components, influence the behavior of motor axons. A combination of histological, biochemical, and molecular biological techniques will be applied to normal and mutant mice, chick embryos, and cultured cells to achieve these aims. Through this work, we hope to gain insight into the retrograde signals that target cells use to determine patterns of innervation.